The present invention relates to a control device for a front and rear wheel drive vehicle and, more particularly, to a control device for a front and rear wheel drive vehicle wherein one of front and rear wheel pairs is driven with an engine and the other one of the front and rear wheel pairs is driven with an electric motor.
In recent years, extensive research and development works have been undertaken to provide a front and rear wheel drive vehicle wherein one of front and rear wheel pairs is driven with an engine and the other one of the front and rear wheel pairs is driven with an electric motor. The front and rear wheel drive vehicle is a vehicle which serves as a hybrid vehicle having a low fuel consumption and which serves as a four-wheel drive vehicle having a running stability.
In general, the front and rear wheel drive vehicle includes a battery that stores electric power to be supplied to the electric motor, and an electric power generator that charges the battery. In a case where the electric motor, which drives the wheels, serves as the electric power generator, the electric motor regenerates a portion of the running energy of the vehicle as an electrical energy, i.e., a regenerative power which is charged into the battery. Usually, the electric motor functions to produce regenerative power during a decelerating condition of the vehicle wherein an accelerator pedal is not depressed. However, in an event that a power remaining capacity level of the battery is below a given power remaining capacity level, a forced charging operation is carried out even when the accelerator pedal is depressed. During regenerative operation of the electric motor, also, a brake force is applied to the wheels associated with the electric motor on account of its regenerative operation.
In addition, the front and rear wheel drive vehicle includes a control device which is arranged to set a drive power of the engine and a drive power of the electric motor and controls the engine and the electric motor in respective control modes. For example, the control modes involve a low xcexc-road running mode for the sake of a running stability at a road with a low frictional coefficient (that will be hereinafter referred to as xe2x80x9cxcexcxe2x80x9d), a fuel saving mode for the sake of fuel consumption and a forced charging mode to be executed when the power remaining capacity level of the battery is lowered, based on which the engine and the electric motor are controlled. More particularly, in the low xcexc-road running mode, a drive power split is carried out in a limited range of power output to be delivered to the road from the respective wheels so as to avoid a slip. In the fuel saving mode, further, the drive power split is carried out so as to attain a high operating efficiency of the engine. In the forced charging mode, also, the drive power of the engine and the amount of electric power to be charged by the electric motor are suitably distributed within a limited range of power output produced by the engine. In this connection, the respective control modes are automatically changed over with the control device and may also be manually changed over by a vehicle driver.
However, if the lowest power remaining capacity level, which serves as a reference to discriminate the forced charging mode to be changed over, remains at a constant value regardless of the road surface conditions, there are some instances where the low fuel consumption of the hybrid motor is degraded. For example, in a case where, when the atmospheric temperature remains at a normal temperature level, the vehicle is continuously ascending the upward slope with a high xcexc-road, since the electric motor also drives the wheels, a gradual decrease will arise in the remaining capacity level of the battery. When this occurs, the power remaining capacity level of the battery decreases to a value below the lowest power remaining capacity level, with a resultant changeover into the forced charging mode. But, in usual practice, a continuous downward slope will appear in a case where the vehicle has passed through the continuous upward slope, and the vehicle encounters a continuous descending phase after a continuous ascending movement. Accordingly, in spite of a probability in that the power remaining capacity level of the battery is expected to be increased due to the regenerative power produced during the decelerating condition of the vehicle at the continuous downward slope, the control device tends to execute the forced charging operation during the continuous ascending operation. When this occurs, since the braking force is applied to the wheels associated with the electric motor during the forced charging operation, the engine tends to operate at a lowered fuel saving efficiency to compensate for such a braking force. In a case where the low xcexc-road is caused to appear due to snow, further, there are many instances where the low xcexc-road will continue for a long time period. In such a case, since an electric power consumption of the electric motor will remarkably increase on account of driving the vehicle in the low xcexc-road running mode, it is required for the power remaining capacity level of the battery to be maintained at an increased level as high as possible. In addition, the lower the battery temperature, the lower will be the performance of the battery to supply electric power. To address this issue, it is necessary for the battery to be charged and the temperature of the battery to be increased to a level as high as possible. However, the presence of the lowest power remaining capacity level of the battery maintained at the constant level regardless of the road surface conditions or the battery performance suffers a problem in that it is difficult to charge the battery at a suitable timing.
It is therefore an object of the present invention to provide a control device for a front and rear wheel drive vehicle wherein regenerative power is produced in dependence on road surface conditions and a battery performance for thereby providing an improved fuel consumption.
According to an aspect of the present invention, there is provided a control device for a front and rear wheel drive vehicle wherein one of a front wheel pair and a rear wheel pair is driven with an engine and the other one of said front wheel and rear wheel pairs is driven with an electric motor with electric power output produced by electric storage means, which comprises drive condition detection means for detecting drive conditions of said front and rear wheel drive vehicle, road surface frictional coefficient estimating means for estimating a road surface frictional coefficient, atmospheric temperature detection means for detecting an atmospheric temperature, temperature detection means for detecting a temperature of said electric storage means, motor drive power calculation means for calculating a drive power of said electric motor on the basis of the running condition detected by said drive condition detection means and the road surface frictional coefficient estimated by said road surface frictional coefficient estimating means, and control mode changeover means for changing over control modes during calculation of the drive power of said electric motor on the basis of the atmospheric temperature detected by said atmospheric temperature detection means or/and the temperature of the electric storage means detected by said temperature detection means.
With such a control device for the front and rear wheel drive vehicle, utilizing the atmospheric temperature as the parameter for changing over the control modes with the control mode changeover means makes it possible to estimate the road surface condition at a high accuracy. Further, the control device utilizes the temperature of the electric storage means as the parameter for changing over the control modes with the control mode change over means, enabling the performance of the electric storage means to be accurately estimated. In addition, the control device changes over the control modes with the control mode changeover means in dependence on the road surface condition and the battery performance, thereby remarkably improving the fuel consumption.
Also, the drive conditions of the vehicle refers to the driving conditions of the front and rear wheel drive vehicle such as the opening degree of the accelerator pedal, the vehicle speed, and the power remaining capacity level of the electric storage means, etc.
In the control device for the front and rear wheel drive vehicle, further, the drive condition detection means comprises the power remaining capacity detection means which detects the power remaining capacity level of the electric storage means, and, when the atmospheric temperature detected by the atmospheric temperature detection means is below the first given atmospheric temperature level, the control mode changeover means settles the lowest power remaining capacity level in dependence on the temperature of the electric storage means, that is detected with the electric storage means temperature detection means, for enabling the drive power of the electric motor to be limited when the power remaining capacity level, detected by the power remaining capacity detection means, is below the lowest remaining capacity level.
With the control device for the front and rear wheel drive vehicle, in a case where the atmospheric temperature remains at the low level below the first given temperature level, since it is estimated that there are many probabilities wherein the performance of the electric storage means is lowered, the lowest power remaining capacity level is allowed to be settled to the high level in dependence on the temperature of the electric storage means. With such a settlement, the electric storage means may have the increased number of frequencies for the forced charging operations and, thus, the power remaining capacity level of the electric storage means increases in dependence on the performance of the electric storage means while raising the temperature of the electric storage means.
Also, the first given atmospheric temperature refers to the low temperature in that snow falls or in that there are many instances where snow falls and, in the preferred embodiment, is exemplarily selected to be 5xc2x0 C. Further, the expression xe2x80x9cto limit the drive power of the electric motorxe2x80x9d refers to the meaning that the operation of the electric motor is interrupted or the electric motor is caused to function as the electric power generator to enable the electric motor to charge the electric storage means.
In the control device for the front and rear wheel drive vehicle, further, the drive condition detection means includes the power remaining capacity detection means which detects the power remaining capacity level of the electric storage means, and, when the atmospheric temperature detected by the atmospheric temperature detection means exceeds the second given temperature level, the control mode changeover means settles the lowest power remaining capacity level in dependence on the road surface frictional coefficient, that is estimated by the road surface frictional coefficient estimating means, for enabling the drive power of the electric motor to be limited when the power remaining capacity level, detected by the power remaining capacity detection means, is below the lowest remaining capacity level.
With such a control device for the front and rear wheel drive vehicle, in a case where the atmospheric temperature remains at the normal temperature level above the second temperature level, since it is estimated that there is a few instances wherein the performance of the electric storage means is degraded and there is the low xcexc-road due to snow, the lowest power remaining capacity level may be settled to the low level in dependence on the road surface xcexc-estimated value. With such a settlement, the electric storage means may have the decreased number of frequencies for the forced charging operations, with a resultant improvement in the fuel consumption at many chances as many as possible provided that the road surface xcexc-estimated value does not remain at the low xcexc-value.
Also, the second given atmospheric temperature refers to the temperature in that there is surely no snow falls and, in the preferred embodiment, is exemplarily selected to be 30xc2x0 C. Further, the expression xe2x80x9cto limit the drive power of the electric motorxe2x80x9d refers to the meaning that the operation of the electric motor is interrupted or the electric motor is caused to function as the electric power generator to enable the electric motor to charge the electric storage means.